This invention relates to the production of arylene sulfide polymer. More particularly, this invention is concerned with the heating and substantial dehydration of a portion of the reactants before the polymerization commences. In another aspect, this invention relates to heating and substantial dehydration under elevated pressure.
As described in U.S. Pat. No. 3,354,129, hereby incorporated by reference, poly(arylene sulfide), more particularly poly(phenylene sulfide), can be produced by first heating under atmospheric pressure a mixture of hydrated alkali metal sulfide in a polar solvent, particularly N-methyl-pyrrolidone, to remove water, particularly H.sub.2 O of hydration, and then heating the resulting mixture with a polyhalo-substituted compound, particularly p-dichlorobenzene, to form an arylene sulfide polymer.
As described in U.S. Pat. No. 3,919,177, hereby incorporated by reference, the initial pre-polymerization mixture to be dehydrated can contain any suitable sulfur source, e.g., Na.sub.2 S; an organic amide, usually N-methyl-pyrrolidone; a base, usually NaOH; and an alkali metal carboxylate, e.g., sodium acetate. The initial pre-polymerization mixture can contain other additives that improve polymer properties such as alkali metal carbonates, lithium halides, lithium borate and the like which are specifially noted in U.S. Pat. Nos. 4,038,259, 4,038,263, and 4,039,518, respectively, all incorporated here by reference.
It is feasible to prepare a pre-polymerization mixture from sodium bisulfide, N-methylamino-butyrate and N-methyl-pyrrolidone, as described in U.S. Pat. No. 3,867,356 hereby incorporated as reference. It is also within the scope of this invention to prepare and dehydrate the pre-polymerization mixture by a two-step process described in U.S. Pat. No. 4,025,496, hereby incorporated by reference.
As described in U.S. Pat. No. 4,064,114, incorporated here by reference, foaming and column flooding are common operating problems of the dehydration, under atmospheric pressure, of pre-polymerization mixtures in an arylene sulfide polymerization process. In addition, the dehydration can result in variable sulfur losses, mainly in the form of hydrogen sulfide generated by the hydrolysis of the sulfur source. These variable sulfur losses can upset the stoichiometric balance of the initial reactants, more particularly sodium sulfide, sodium hydroxide, N-methyl-pyrrolidone and, optionally, sodium acetate, and can result in arylene sulfide polymer having undesirably low molecular weight (high flow rate) and/or in excessively low polymer yields. The effective control of sulfur losses during dehydration is essential in producing poly(arylene sulfide) that meets narrow flow rate and property specifications, at high yields. It is theorized, though not wishing to be bound by this theory, that the composition of the polymerizable complex formed and the extent of its formation during the heating and dehydration step prior to the addition of the polyhalo-substituted compound have a crucial effect on the subsequent polymerization reaction.
A method has now been discovered for dehydrating pre-polymerization mixtures under elevated, declining pressure conditions which results in the following improvements as compared to dehydration under essentially atmospheric pressure conditions: (a) reduced foaming of the heated mixture, controlled further by slight, temporary pressure increases which can be achieved more effectively than cooling the reactor; (b) reduced flooding of the distillation column, again controlled further by slight, temporary pressure increases; (c) more consistent and less extensive sulfur losses resulting in production of fewer off-specification polymer batches; (d) more rapid dehydration resulting in a shorter cycle time, i.e., time necessary to produce a poly(arylene sulfide) batch; and (e) shortened cycle time and reduced column flooding problems as compared to a process requiring controlled, elevated, constant pressure dehydration.
It is, therefore, an object of this invention to provide a method for dehydrating pre-polymerization mixtures in the preparation of poly(arylene sulfide). It is another object of this invention to provide dehydrated pre-polymerization mixtures using the process described herein. It is still another object of this invention to provide a method for polymerization of the dehydrated pre-polymerization mixtures produced by the present invention.
Other aspects, objects and the various advantages of this invention will become apparent upon reading this specification and the appended claims.